Tomcat类装载的问题.

昨天发布一个web application的时候发现一个奇怪的问题, 明明jar包里面有的类说找不到, 真是奇怪. 不过要下班了. 明天再说吧.

java.lang.NoClassDefFoundError: com/netsboss/util/dbobject/GeneralEntity
  java.lang.ClassLoader.defineClass1(Native Method)
  java.lang.ClassLoader.defineClass(ClassLoader.java:620)
  java.lang.ClassLoader.defineClass(ClassLoader.java:465)
  com.netsboss.util.proxy.ProxyClassLoader.defineClass(ProxyClassLoader.java:75)
  com.netsboss.util.proxy.Proxy.getProxyClass(Proxy.java:216)
  com.netsboss.util.dbobject.DatabaseManager.generateClass(DatabaseManager.java:1161)
  com.netsboss.util.dbobject.DatabaseManager.<init>(DatabaseManager.java:240)
  com.netsboss.util.dbobject.DatabaseManager.getDatabaseManager(DatabaseManager.java:98)
  com.aft.constants.SystemConstants.getDefaultAFTDatabaseManager(SystemConstants.java:258)
  com.aft.database.retrieve.CAFTDBGetWebUserInfo.<init>(CAFTDBGetWebUserInfo.java:64)
  com.aft.database.retrieve.CAFTDBGetWebUserInfo.getCAFTDBGetWebUserInfo(CAFTDBGetWebUserInfo.java:51)
  com.aft.servlets.actions.RegisterAction.perform(RegisterAction.java:43)
  org.apache.struts.action.Action.execute(Action.java:420)
  org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:484)
  org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:274)
  org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
  org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:525)
  javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
  javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

上面的是异常的信息, 我明明把含有这个类的jar包放到了WEB-INF/lib下面. 怎么办,还能这么办, trace code. 
结果发现在了一个自定义的类装载器com.netsboss.util.proxy.ProxyClassLoader,他在构着函数里面没有传他的parent classloader(tomcat Webapp classloader)进去, 所以他的parent classloader是systemclassloader, 我想假如ProxyClassLoader的构着函数里面调用了super(ProxyClassLoader.class.getClassLoader());的话,
 应该能解决这个问题的. 我想这个就是这个java.lang.NoClassDefFoundError异常的原因吧.     

这里有篇介绍类装载的问题, 也介绍了Tomcat类装器的机制.
http://blog.csai.cn/blog/number01/2005628165135.html

添加一些: 

这个是自定义的classloader, 他在构着函数里面没有传他的parent classloader(tomcat Webapp classloader)进去, 所以他的parent classloader是systemclassloader,

具体代码看java.lang.ClassLoader. 

    /**
     * Creates a new class loader using the specified parent class loader for
     * delegation.
     *
     * 
 If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * checkCreateClassLoader} method is invoked.  This may result in
     * a security exception.  

     *
     * @param  parent
     *         The parent class loader
     *
     * @throws  SecurityException
     *          If a security manager exists and its
     *          checkCreateClassLoader method doesn&apost allow creation
     *          of a new class loader.
     *
     * @since  1.2
     */
    protected ClassLoader(ClassLoader parent) {
 SecurityManager security = System.getSecurityManager();
 if (security != null) {
     security.checkCreateClassLoader();
 }
 this.parent = parent;
 initialized = true;
    }

    /**
     * Creates a new class loader using the ClassLoader returned by
     * the method {@link #getSystemClassLoader()
     * getSystemClassLoader()} as the parent class loader.
     *
     * 
 If there is a security manager, its {@link
     * SecurityManager#checkCreateClassLoader()
     * checkCreateClassLoader} method is invoked.  This may result in
     * a security exception.  

     *
     * @throws  SecurityException
     *          If a security manager existsand its
     *          checkCreateClassLoader method doesn&apost allow creation
     *          of a new class loader.
     */
    protected ClassLoader() {
 SecurityManager security = System.getSecurityManager();
 if (security != null) {
     security.checkCreateClassLoader();
 }
 this.parent = getSystemClassLoader();
 initialized = true;
    }

package com.netsboss.util.proxy;

import java.io.File;

import java.io.FileOutputStream;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

import java.net.URL;
import java.net.URLDecoder;
import java.net.URLEncoder;

import java.util.HashMap;
import java.util.Locale;

import org.apache.log4j.Category;

public class ProxyClassLoader extends ClassLoader {

 private static Category log = Category.getInstance(ProxyClassLoader.class);

 private HashMap classes = new HashMap();

 private boolean saveToClassPath = false;

 public ProxyClassLoader(boolean saveToClassPath){

  this.saveToClassPath = saveToClassPath;

 }

 /**

  * @see ClassLoader#findClass(String)

  */

 protected Class findClass(String name) throws ClassNotFoundException {

  Class find = super.findClass(name);

  if(find != null) return find;

  find = (Class)classes.get(name);

  if(find != null) return find;

  else throw new ClassNotFoundException(name);

 }

 public void defineClass(byte classfile[], String classname){

  Class clazz = defineClass(classname, classfile,0, classfile.length);

  classes.put(classname,clazz);

  if(saveToClassPath){

   saveToClassPath(classfile, classname);

  }

 }

 /**

  * return a class which implements all the interfaces or inherit from one of class

  * currently, just return null.

  */

 public Class search(Class interfaces[]){

  return null;

 }

 private void saveToClassPath(byte classfile[], String classname){

  String filename = classname + ".class";

  URL url = getClass().getProtectionDomain().getCodeSource().getLocation();

  if( "file".equals(url.getProtocol()) && (url.getHost() == null || url.getHost().equals("")) ){

      String dir = null;

      try{
        dir = URLDecoder.decode(url.getFile(), "UTF-8");
      }catch(UnsupportedEncodingException ex){
        log.error("Encoding not supported", ex);
      }

   File classpath = new File(dir);

   if(classpath.isDirectory() == false){

    log.info("Classpath error for ProxyClassLoader:" + dir + " not a directory");

    return;

   }

   File file = new File(classpath, filename);

   try {

    FileOutputStream os = new FileOutputStream(file);

    os.write(classfile);

    os.close();

   }

   catch(IOException ex){
        log.error("Error writing Proxy Class file", ex);
    ex.printStackTrace();

   }

  }

 }

}

参考这篇文章

http://www.cjsdn.net/post/view?bid=29&id=162060&sty=1&tpg=1&age=0